24 4/11/98 CSE 143 Stream I/O [Appendix C]

25 4/11/98 Input/Output Concepts  Concepts should be review!  New syntax, but same fundamental concepts  input vs. output, read vs. write  file; file name vs. file variable  open; close  end-of-file  conversion from character steam to C/C++ types

26 4/11/98 Stream I/O  C++ uses the concept of streams for I/O  both for keyboard/monitor and for files  C++ can also use old C-style printf, scanf, etc.  Mixing the two is not recommended  You must use only stream I/O in CSE143

27 4/11/98 What is a Stream?  Think of a stream as a sequence of characters  istream and ifstream are types of input streams  Keyboard ( istream )  File ( ifstream )  ostream and ofstream are types of output streams  Screen ( ostream )  File ( ofstream )

28 4/11/98 Streams in Pictures A stream is just a sequence of characters: Smythe, J someIstreamInstance Input cursor Welcome to someOStreamInstance Output cursor C++ Program input via >> operator output via << operator 64

29 4/11/98 Well-Known Streams  Global streams defined in iostream.h :  cin : standard input stream (usually keyboard)  cout : standard output stream (usually screen)  cerr : standard error stream  Programs can open other streams to/from files and other devices:  fstream.h : for input/output with files  strstream.h : for treating strings as streams

30 4/11/98 Stream Output Operation For output streams, << is the “put to” or "insertion" operator #include … int count = 23; cout << “Hello, World!” << endl; // endl equivalent to “\n” cout << “The count is ” << count << endl;

31 4/11/98 Stream Input Operation For input streams, >> is the “get from” or "extraction" operator #include … int x, ID; char Name[40]; cin >> x; cin >> Name >> ID; // Can read multiple items on one line // Note: no &’s as needed with scanf

32 4/11/98 How Stream Input Works Input stream characters are interpreted differently, depending on the data type: int ID; char Name[40]; char ch; cin >> ID; // interprets as integer cin >> ch; // reads a char, skipping whitespace cin >> Name; // interprets as character string, // skipping leading whitespace and //stopping at trailing whitespace

33 4/11/98 Other Stream Operations  Sometimes want to read from a file without ignoring whitespace (spaces, tabs, newlines) get(c);// reads one character into c  Get also returns a value which we can test to see if the operation succeeded. (It would fail, for instance, if the end-of-file was reached)  There are several varieties of get() and other I/O operations. See Appendix C and Lippman for details.

34 4/11/98 File Stream Example #include void main() { ifstream inFile("input.txt");// open input ofstream outfile("output.txt");// open output char ch; // should test for successful opening here.. while (inFile.get(ch)) {// while more input outFile.put(ch);// write it to output } inFile.close();// close the files... outFile.close(); }

35 4/11/98 Preview of C++ Classes  Streams are instances of C++ classes  Class instances are like structs in that they have fields (generally called members).  Members can be data or functions (methods)  We use the “.” syntax to access members. Eg: inFile.get(ch);// get a character outFile.put(ch);// put a character outFile.close();// close the stream outFile.eof();// end of File??

36 4/11/98 Stream Testing  We often want to test the status of a stream, to see if it has any more data.  It’s usually cleanest to use the return value of the input function … while (cin.get(ch)) {... } while (cin >> someInt) { // if the get succeeded // do something }

37 4/11/98 Stream Testing (2)  It’s also possible to use the eof() member function: if (!someFile.eof()) {... }  For keyboard input, special characters such as Ctrl-Z (on most PCs) or Ctrl-D (on UNIX) signal end-of-file

38 4/11/98 Stream Advantages  Type Safety  No more dangers of mismatched % -types scanf("%s", &someInt); // oops! scanf("%d", someInt); // dooh!  Readability  Easier to interpret than a sequence of % -types scanf("%d%c%d", &intOne, &oneChar, &intTwo); versus cin >> intOne >> oneChar >> intTwo;